1. Field of Invention
The present invention relates to paper feed apparatuses provided with a pickup roller that performs sheet-by-sheet feeding of originals which have been placed on an original placement tray.
2. Related Art
In conventional image forming apparatuses provided with original reading apparatuses, continuous reading of wads of different-size originals is prohibited. That is to say, it is prohibited to place wads of originals including different-size originals on an original placement tray. This is in order to prevent diagonal paper feeding by the pickup roller due to continuous paper feeding by the pickup roller of different-size originals.
And in order to very reliably prevent occurrences of diagonal paper feeding, in recent image forming apparatuses, as shown in FIG. 8, an original placement tray 201 is constructed so that originals are placed at a center reference L0 in the tray-width direction. That is, a pair of paper pressing plates 202, 202 is arranged on left and right sides perpendicular to a paper transport direction X and these paper pressing plates 202, 202 can be moved closer or farther from each other by a rack-and-pinion 203. This ensures that the paper pressing plates 202, 202 always move to equivalent lateral positions with respect to the center reference L0. For this reason, when the paper pressing plates 202, 202 are moved to align with the width size of the papers to be placed, a width-direction center position of the papers to be placed here is always made to match the center reference L0 of the original placement tray 201. Thus, regardless of the size of the originals placed on the original placement tray 201, the originals that are placed are placed in such a manner that the center reference L0 always becomes the width-direction center position.
On the other hand, a pickup roller 210 and a separation roller 211 are arranged so as to have lateral symmetry with respect to the position of the center reference L0. Thus, regardless of the size of the originals placed on the original placement tray 201, the pickup roller 210 can perform stable feeding of originals by making contact therewith with an equivalent lateral-direction width from the width-direction center position of the originals that have been placed, and therefore the possibility of diagonal paper feeding occurring is low.
In this regard, along with the advancement of developments in high-speed image forming apparatuses in recent years, “same-time reading of different-size originals” has come to be allowed while providing a predetermined condition. Here, “predetermined condition” is a limit in mixed type originals of a small size original being an original size one rank smaller than a large size original. Specifically, these are combinations such as A3 with B4, A4 with B5, B5 with A5, and B4 (landscape) with A4 (portrait).
However, when paper feeding is carried out by placing wads of originals, in which different sizes are mixed, on the original placement tray 201 having the center reference L0, large size originals are pressed on their lateral edges by the paper pressing plates 202, 202 on the left and right and therefore are aligned with the center reference with respect to the pickup roller 210, but small size originals are usually placed with one edge thereof aligned with the large size originals and therefore are not positioned on the center reference with respect to the pickup roller 210. For this reason, the pickup roller 210 makes contact with the small size originals in a position displaced from the width-direction center position of the small size originals and therefore there is a problem of the likelihood of diagonal paper feeding since a laterally equivalent load is not applied to the small size originals. This may result in a range of problems such as tearing of the originals due to the originals impacting on an end face of a paper guide (omitted in drawings) in the transport path thereafter and defective image reading due to diagonal reading during reading the original.
Consequently, automatic paper feed apparatuses (for example, see JP H11-79435A (hereinafter referred to as “Patent Document 1”)) and original transport apparatuses (for example, see JP H08-282880A (hereinafter referred to as “Patent Document 2”)) have been proposed to address the problem of diagonal paper feeding (that is, diagonal transport).
The automatic paper feed apparatus described in Patent Document 1 is configured such that two assistive rollers are arranged on the sides in substantially equivalent positions to a feeding roller or on an upstream side of the feeding roller with respect to the paper feeding direction of the originals, and they sandwich the feeding roller in a direction perpendicular to the paper feeding direction. The assistive rollers are then rotated with no driving force being applied in the same direction as the feeding roller and apply pressure to the surface of the originals prior to the feeding roller landing on the surface of the originals.
Furthermore, the original transport apparatus described in Patent Document 2 is provided with a transport roller provided at a leading edge position of the originals that have been placed and a pressure roller, which is positioned on an upstream side of the transport roller and applies pressure to the originals during transport of the originals.
That is to say, in the configurations provided in Patent Documents 1 and 2, occurrences of diagonal movement are prevented by applying pressure to the originals more strongly using assistive rollers or a pressure roller during feeding of the originals or during transport of the originals.
However, in both of these apparatuses described in the conventional art, there is a necessity to arrange new roller members such as the assistive rollers and the pressure roller, and therefore there are problems in that there is a necessity to cordon locations for those arrangements and the structures become complicated structurally. Furthermore, neither of the apparatuses described in the conventional art give any consideration to continuous reading by placing wads of originals having different sizes on the original placement tray and accordingly have the problem of being configurations not suited to a case of reading wads of different-size originals.